Genotypic Survey of Calonectria pseudonaviculata Isolates from Oregon Boxwood Operations with an Assessment of Phenotypic Response to Temperature

Boxwood blight disease, caused by the fungal pathogen Calonectria pseudonaviculata (Cps), is a significant threat to the United States nursery industry, where Oregon is the leading producer. While current knowledge of Cps variation is primarily derived from studies of eastern U.S. populations, the genetic and phenotypic variation in western populations, particularly their adaptation to regional climates, remains poorly understood. To address this, the genetic diversity of 123 Oregon isolates was evaluated through the sequencing of eleven simple sequence repeat (SSR) loci and the influence of temperature on mycelial growth was assessed across a subset of 71 representative isolates at five temperatures (10 - 30°C). Genotyping identified nine multi-locus genotypes (SSR-MLGs), including five novel genotypes. While 89% of isolates belonged to previously identified SSR-MLGs, the G1 genotype occurred at a higher frequency (20%) than previously reported in populations outside of Oregon. One of the new SSR-MLGs occurred at a frequency of 77% among samples collected from a single nursery population. The temperature assays revealed that optimal growth for most Oregon isolates (69 of 71) occurred at 20°C which is cooler than the 25°C standard reported for isolates from other regions. These findings indicate that Oregon Cps populations may be genetically distinct from eastern populations and are adapted to cooler temperatures. This highlights the potential impact of regional differences on growth and disease development across the U.S. and the need to integrate these differences into predictive modeling and disease management strategies.